Drawing Out the Facts

The STEM Chicksmas Day Three: Nanoscopy

For the 12 Days of “The STEM Chicksmas” we’re highlighting 12 scientists who have contributed something innovative and exciting to their field. It is the season of giving, and these brilliant minds have given incredible gifts to the scientific community! This year we’re looking at 12 Nobel Prize winners from the past 15 years in the fields of Physics, Chemistry, and Physiology or Medicine.

The invention of the optical microscope by Antonie van Leeuwenhoek kick started the field of microbiology by providing scientists in the 17th century a first look into the rich world of microorganisms. Almost 400 years later, we have improved our microscopy techniques to resolutions Leeuvenhoek would not have been able to imagine, however this resolution remains limited. In the late 1800s microscopist Ernst Abbe demonstrated that an optical microscope could never have a higher resolution than half a wavelength of light, or around 200 nanometers, due to the properties of diffraction. Unfortunately, many subcellular structures and nanoparticles in general are below this limit, so they cannot be imaged with optical microscopy, which uses lenses and the diffraction of light to enhance an image. Other types of microscopy, such as scanning electron microscopy, can be used to obtain much higher resolution images at these scales, but these types are not compatible with living organisms. Biological samples also must be dry to be looked at in scanning electron microscopy, and because many biological systems have water as a part of the system, the dried sample is very different from the real sample.

However, the 2014 winners in chemistry independently developed a clever way to circumvent this limit. Stefan Hell, W.E. Moerner, and Eric Betzig were all interested in using fluorescence to get around this limit.

Hell came up with the idea of the Stimulated Emission Depletion (STED) microscope. The way this works is that you can take a biological sample that is able to fluoresce, or emit light when excited to a certain energy level. You use a laser bean to excite all of the molecules on your sample, and then use a second beam to deexcite all of them except those in a very small area. You take an image of this fluoresced area, and then move onto the next area and repeat the process. At the end, you have images of every part of the sample and you can piece them together for a very high resolution image. This overcame Abbe’s diffraction limit by essentially working around it. Hell used this technique to generate a very high resolution image of E. coli.

W.E. Moerner was the first to measure the fluorescence of a single molecule. He later expanded on this discovery by dispersing fluorescent molecules in a homogenous medium, like a gel. Each of the molecules was further than 200 nm apart, and when they were all excited the fluorescence of each could be detected, like individual fireflies.

Moerner’s work inspired Betzig, whose technique we highlight in our graphic above. Beztig’s group coupled, or attached, fluorescent proteins to a lysosome. They then used a weak pulse of light, which caused only some of the proteins to fluoresce. (The intensity of the light was too weak to fluoresce all of them.) Because of probability, these proteins generally ended up being further apart from each other than 200 nm, so they were each detectable, as Moerner had previously demonstrated. The researcher would take a picture of the glowing proteins, wait for the fluorescence to stop, and then apply another weak pulse. This time, a different group of proteins would fluoresce. They did this over and over, and at the end they superimposed all of the images. Like Hell’s work, this led to a very high resolution image. However, where Hell treated his images like a puzzle, Betzig treated his like individual layers that at the end revealed a whole image.

These scientists’ work demonstrated that it’s possible to optically image very small systems. They’ve opened up the way to looking more deeply than ever into cells, and the atom’s the limit. Maybe in another 400 years scientists will open up a world that is just as unimaginable to us as the one Hell, Moerner, and Betzig helped reveal was to Leeuwenhoek and his contemporaries.

To read more, check out the Nobel Prize website here or read the original Nobel Prize winning work here (Hell), here (Moerner), or here (Betzig). Please note that each researcher published several articles that contributed to his prize and the papers linked above are samplings. A more complete list can be found in the references section here.